Other mistake: Grand-Santa has made 70 missions, and Santa (Malcolm) has just completed his 70th mission, totaling 140 years. Yet Grand-Santa is only 136 years old.
Suggested correction: Grandsanta's portrait is shown in the halls of the North Pole with the dates 1902-1941 (with Malcom having done 71 missions from 1941). Therefore he did 39 missions. That makes for a total of 109 years, meaning he was 27 when he took over from his own father.
Character mistake: When they deliver Xmas presents to Denmark it's already too late - Denmark has the presents on the 24th, early in the evening.
Continuity mistake: Arthur's older brother gets "bombed" by a seagull. In the very next scene, there is nothing on his jacket, but the offending "message" appears again a little later on.
Continuity mistake: Arthur runs after Steve with the Santa game piece and bursts through the swinging doors which close behind him. Yet, the doors are open again when they are talking and Steve tells him to shut the doors.
